In the design and construction of various types of machinery, closely fitted rotating parts are often found which must run together or slide past one another without galling or experiencing unacceptable wear. In many cases, materials such as the lead-tin babbits can be selected and when coupled with a suitable lubricant, low frictional forces and low rates of wear are attainable. When the lubricant layer is sufficiently thick and maintained to prevent material contact, a state of hydrodynamic lubrication exists. When the film is not sufficient to keep the mating materials completely separated and some contact occurs, boundary lubrication exists.
In many cases, it is not possible to select "bearing" type materials for mating parts, and the use of suitable lubricants is not possible. One of the most common types of machinery in this category is pumps. Most centrifugal type pumps, which contain rotating impellers, require close tolerances (0.010 inch to 0.020 inch diametrical clearance) between the impeller hub and the casing to prevent leakage which can decrease the efficiency. During transient periods, such as starting and stopping, there can be contact between the impeller and the casing, particularly in multistage pumps where some deflection of the shaft occurs at rest. Unfortunately, these parts sliding past one another must depend upon the lubricating ability of whatever fluid is being pumped. In many cases, these fluids are not good lubricants.
The most common technique employed to prevent galling and unacceptable wear of these components is the use of impeller and casing wear rings, where "compatible" materials are selected. For example, one can use a material like cast iron, where the graphite flakes act as a built-in lubricant. Another technique is to harden materials so that there is at least a 50 Brinell hardness spread between the parts or to harden both components above 400 Brinell, where the hardness differential is not required. Obviously, this technique of hardening will only work on materials which can be hardened, such as steels having sufficient carbon, or with coatings. However, the corrosiveness of many fluids, such as seawater or brines containing hydrogen sulfide, precludes the use of hardenable materials and in many cases, coatings. Unfortunately, most corrosion resistant materials, such as the austenitic stainless steels and the nickel based alloys, have very poor wear characteristics and will gall if contact occurs. Although it is possible to improve the wear characteristics of some of these corrosion resistance materials with weld overlays, the process is expensive and in some cases the corrosion resistance of the base material can be destroyed.
It is therefore an object of this invention to provide a corrosion and wear resistant alloy which exhibits the combination of corrosion and wear resistance to an extent not heretofor obtainable in commercial alloys of reasonable cost.
This and other objects are obtained in corrosion and wear resistant alloy comprising the following anticipated ranges of critical elements:
______________________________________ C Mn Si P S Cr ______________________________________ % MIN. -- -- -- -- -- 20.0 % MAX. 0.08 1.0 1.0 0.03 0.03 25.0 ______________________________________ Mo Fe Al Ti Bi Sn Sb Ni ______________________________________ % MIN. 6.0 -- -- -- 2.0 2.0 1.0 -- % MAX. 10.0 5.0 0.4 0.4 5.0 5.0 3.0 Balance ______________________________________